feat: add landing page + builtins Worker + BETA_TEST guide + README

- landing/: Next.js 15 app for arcrun.dev (dashboard, integrations,
  API docs, login). Deploys via Cloudflare Pages — CI scan skips
  this via pages_build_output_dir marker.
- builtins/: minimal Hono Worker at arcrun-builtins (/init for
  one-shot component registry seeding). initComponents logic is
  flagged stale in src/index.ts for future rewrite.
- BETA_TEST.md: pre-launch validation playbook.
- README.md: updated to match current arcrun.dev / acr CLI flow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

builtins/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "arcrun-builtins",
+  "version": "1.0.0",
+  "private": true,
+  "scripts": {
+    "dev": "wrangler dev",
+    "deploy": "wrangler deploy"
+  },
+  "dependencies": {
+    "hono": "^4.7.0"
+  },
+  "devDependencies": {
+    "@cloudflare/workers-types": "^4.20250219.0",
+    "typescript": "^5.7.0"
+  }
+}